1. Field of the Invention
The present invention relates to a pulse based communication system, more particularly relates to a synchronization acquisition method in an ultra wide band (UWB) or other pulse based communication system.
2. Description of the Related Art
A UWB system transfers signals in a pulse state without using carriers. The Federal Communications Commission (FCC) of the U.S. approved its use in the American private sector in 2002. The FCC defines UWB as “a wireless signal having a bandwidth of at least 500 MHz or an occupied bandwidth of at least 20% of the center frequency”. UWB is the focus of much attention as technology enabling high speed communication. A UWB system has the merit of enabling sharing of the spectrum with other communication systems since it uses an extremely wide bandwidth and transmits with a low power. Further, the system transmits signals by pulses, so can eliminate the effects of multipath or fading (see M. Z. Win, R. A. Scholtz, “Ultra-Wide Bandwidth Time-Hopping Spread-Spectrum Impulse Radio for Wireless Multiple-Access Communications”, IEEE Trans. on Commun., vol. 48, no. 4, April 2002, and K. Siwiak, P. Withington, S. Phelan, “Ultra-wide band radio: the emergence of an important new technology,” Vehicular Technology Conference, VTC 2001 Spring IEEE VTS 53rd, vol. 2, pp. 1169-1172, May 2001).
The short pulse UWB waveform enables the effects of multipath to be avoided, so is applicable to indoor communication. The following description envisions a multipath-free addition type white noise AWGN model.
As described in Fernando Ramirez-Mireles, “On the Performance of Ultra-wide-Band Signal in Gaussian Noise and Dense Multipath,” IEEE Trans. on Vehicular Technology, vol. 50, no. 1, January 2001, the waveform w(t) of a monocycle is given by the following equation (1):
                              w          ⁡                      (            t            )                          =                              {                          1              -                              4                ⁢                                                      π                    ⁡                                          (                                              t                                                  τ                          m                                                                    )                                                        2                                                      }                    ⁢          exp          ⁢                      {                                          -                2                            ⁢                                                π                  ⁡                                      (                                          t                                              τ                        m                                                              )                                                  2                                      }                                              (        1        )            where, τm is the magnitude of the pulse width
FIG. 11 is a view of an example of the waveform of a transmission signal modulated by a direct sequence (DS) scheme. In this example, S number (S is a positive integer) of successive pulses of a pulse string of a period Tf express 1 bit of information. For the modulation system, TH (time hopping)-UWB is known in addition to the DS shown in FIG. 11 inverting the phases of the pulses.
FIG. 12 is a view explaining the DS-UWB modulation system. As shown in the figure, with DS-UWB, the pulses of the pulse string are inverted to define 0 and 1. This is similar to the binary phase shift keying (BPSK) technology in conventional CDMA defining information 1 and 0 by noninversion and inversion of phase of the carrier.
FIG. 13 is a view for explaining the TH-UWB modulation system. As shown in this figure, with TH-UWB, positions of the pulses on the axis are shifted for example 125 picoseconds and 0 and 1 are defined by those positions.
In each modulation system, as explained above, S number (S is a positive integer) of successive pulses (PN code or Baker code) are spread in spectrum to express 1 bit of information.
In a DS modulation or TH modulation UWB system, the synchronization of the code is an important issue. In many cases, synchronization acquisition is necessary in the state with a very low SN ratio or in the presence of interference waves.
In spread spectrum communication in the DS modulation or TH modulation UWB system, synchronization acquisition methods of the related art include the synchronization acquisition method using a matched filter and the synchronization acquisition method using the correlation method.
The synchronization acquisition method using a matched filter enables fast synchronization acquisition, but the hardware becomes large in size.
The synchronization acquisition method using the correlation method is relatively simple in terms of hardware, but ends up taking time for synchronization acquisition.
To shorten the time for synchronization acquisition, there is the method of synchronization acquisition using a plurality of correlators. However, this method results in a complicated receiver and increases the power consumption.
As a general method of initial synchronization by the correlation method, there is the method of successively changing the possible state of phases until the correct code phase is obtained. Each phase is evaluated as to whether it is correct by trying out the despread in the received signal. If the estimated code phase is correct, despread is performed and the correlation peak is detected. Further, if the estimated code phase is not correct, despread is not performed and the reference signal is changed to the next phase for the next estimation. This technique is called a “serial search” (see Roger L. Peterson, Roger L. Ziemer, David E. Borth, Introduction to Spread Spectrum Communications, Prentice-Hall 1995).
However, according to this serial search method in the correlation method, there is the problem that the longer the absence of information in the transmission signal continues, the longer the time required for synchronization acquisition at the reception side (for other publications on the related art, see Kazimierz. SIWIAK, “Ultra-wide Band Radio: Introducing a New Technology,” Vehicular Technology Conference, VTC 2001 Spring, IEEE VTS 53rd, vol. 2, 6-9 May 2001 and Jack K. Holmes, “Acquisition Time Performance of PN Spread-Spectrum Systems,” IEEE Trans. Commun., COM-25, 8, pp. 778-783, August 1977).